The polyclonal antibody response of mouse B lymphocytes to anti-immunoglobulin and nonantigen-specific soluble factors derived from activated T cells or macrophages is being studied as a model for the thymus-dependent antibody response. We are studying the roles of cell surface IgM cell surface IgD, and soluble factors in the regulation of B cell proliferation and differentiation to high rate Ig secretion, which appear to be separately regulated.